Micro-sized liquid or solid particles suspended in a gas are generally called aerosols. Most of the contaminants contained in the exhaust gas of automobiles or in the smoke emitted from manufacturing plants are aerosols. In particular, aerosols with a particle diameter smaller than 1 μm, or so-called “nano-aerosols”, have raised concerns about their unfavorable influences on health. Therefore, measuring their particle diameters or distribution of particle diameters has been extremely important in such areas as environmental measurement and assessment. As a device for measuring the particle-diameter distribution of aerosols (particle classifier), a differential mobility analyzer (DMA), which classifies microparticles using the difference in the moving speed of charged microparticles within an electric field (electric mobility), has been popularly used (see Patent Literature 1 or other documents).
In a measurement using a DMA, the particles to be subjected to the measurement (aerosols) need to be electrically charged in advance of the measurement. To this end, several types of particle chargers have been conventionally used. A comparatively traditional type of particle charger is one which uses alpha rays emitted from americium (Am), beta rays emitted from krypton (Kr) or similar radiations as the ion source and forces the ions generated by this ion source to come in contact with the particles as the charging target to electrically charge those particles (see Non Patent Literature 1). This device is also called the “bipolar diffusion neutralizer”, or simply “neutralizer”.
However, this type of particle charger has problems associated with the use of the radiation source, such as the necessity of being operated with the greatest concern for safety as well poor portability. Accordingly, in recent years, as a substitute for this particle charger, a particle charger using an ion source which employs electric discharge, such as corona discharge, has been increasingly used. For example, as described in Patent Literature 2, this type of particle charger ionizes an appropriate kind of carrier-gas molecules by corona discharge or similar electric discharge and forces the generated ions to come in contact with sample particles as the charging target to electrically charge those particles.
FIG. 8 shows one example of the configuration of a particle classifying and observing system including the previously described type of particle charger along with a particle classifier. In this particle classifying and observing system, the particles (aerosols) as the measurement target collected with a sampler 100 are sent into the particle charger 200, which electrically charges those panicles and sends them to the particle classifier 300. The charged particles sent to the particle classifier 300 are classified according to the difference in their electrical mobilities. After that, those particles are sent to a particle counter 400 so as to be counted per particle diameter, or are collected with a collector 500 so as to be observed through an observing device 600.